In many conventional electron spectrometers, an electrostatic lens called as an input lens is used in an electron acceptance section of energy analyzer (typically, electrostatic semi-spherical analyzer). The input lens has functions of (i) accepting, as many as possible, electrons emitted from a specimen, (ii) decelerating the electron, and (iii) introducing the thus decelerated electron into the analyzer. In this way, the input lens improves energy resolution.
Moreover, some electron spectrometers are provided with a function of limiting a field of view through which the electron is accepted from a surface of the specimen. In the electron spectrometers having such an arrangement, their sensitivity is determined by how widely an angle is opened through which the electrons are accepted and introduced to the input lens. Moreover, an energy analyzer having a function of forming an image can, by forming the image from distribution of acceptance angles, measure angle dependency of an energy peak of photoelectrons at the same time when forming the image. In this case, as long as the acceptance angle is 90° or more, it is possible to measure at once angle dependencies for angles within a range of from an angle just off the surface to an perpendicular angle. Thus, this realizes efficient measurement of relationship between elements of a specimen and depth of the specimen.
However, a general electrostatic lens cannot focus, into one point, beams within a wide opening angle due to spherical aberration. Specifically, the acceptance angle for the general electrostatic lens is limited to be about ±20° or less.
Moreover, in a general photoemission electron microscope, a wide acceptance angle is realized by arranging such that photoelectrons and secondary electrons emitted from the specimens are accelerated and then introduced into an object lens. However, there are some cases that the acceptance angle becomes smaller because the electrons with large emission energy curve less. Specifically, when the emission energy is several hundred eV, the acceptance angle is about 30° (±15°) or less. If it becomes possible to perform measurement with a wide solid angle under the conditions that the emission energy is set at several hundred eV or more, this enables structural analysis of atomic arrangements, such as photoelectron diffraction and photoelectron holography. However, the acceptance angle of about 30° or less is insufficient for performing the structural analysis of atomic arrangements.
Furthermore, it has been proved that spherical aberration inevitably occurs in an electron lens, and that zero spherical aberration cannot be attained in a general lens configuration, in which a special electric charge in axial symmetry is not present. This prevents realization of a large acceptance angle. In view of this, there is an approach in which a mesh electrode or a foil electrode is provided on the way to the lens. This arrangement provides an effect equivalent to application of the special electric charge, thereby correcting the spherical aberration.
In case where the foil is used, it is necessary that the energy of the electrons be set high to some extent so that the electron beam can pass through the foil. It is possible to set the energy of the electors high in transmission electron microscopes or the like. However, it is difficult to set the energy of the electrons high in electron spectrometers which are used for measuring electrons having an energy of several keV at most.
Moreover, there is such a problem that it is difficult to shape the foil to have a curved surface, because, even if the electrons have a high energy, the foil should be sufficiently thin in order to prevent scattering of the electrons and the absorption of the electrons. Note that up to third order spherical aberration can be reduced to zero with a flat foil but it is difficult for the flat foil to cancel out higher order spherical aberration. In the electron microscope, high resolution is attained by narrowing the opening angle of the electron beams to an order of mrad. Thus, it is sufficient in the electron microscope that at least the third order spherical aberration can be corrected. However, the foil is not an effective means for correcting the spherical aberration for the beams within an opening angle of several tens degrees, which is required in electron spectrometers.
The aforementioned problems associated with the correction using the foil can be solved by using a mesh in lieu of the foil. The use of the mesh solves the problem in the transmission property. Further, compared with the foil, it is easier to shape the mesh to give it a curved surface. Conventionally, as described in Japanese Patent Application, Tokukaihei, Publication No. 8-111199 (published on Apr. 30, 1996), use of a spherical mesh is proposed to improve the aberration (see FIG. 8). The use of the spherical mesh attains an acceptance angle as high as ±30°.